The present invention relates to a highly heat-resistant porous material for use in construction materials, heat insulators and insulating materials and a method for producing the same.
Polyimide resins exhibit excellent heat resistance and superb insulation and therefore enjoy a wide application for liquid crystal alignment films, interlayer insulating films for semiconductor circuits, and the like. Active use of such advantages of polyimide resin and realization of a product exhibiting high performance by substituting the polyimide resin for a conventionally used resin have been investigated. Addition of a new function to the polyimide resin to a high addition value in the polyimide resin has also been studied.
One exemplary technical development in the polyimide resin is an attempt to reduce the dielectric constant or thermal conductivity of polyimide resin by forming the polyimide resin into a porous material. The technology for making the polyimide resin porous has been disclosed in, for example, the Japanese Laid-Open Patent Publication No. Hei 5-205526 and Thermal Conductivity, Vol. 18, pp. 437-443, 1985 (by D. D. Barley, entitled xe2x80x9cThermal Conductivity of a Polyimide Foamxe2x80x9d).
The Publication No. Hei 5-205526 discloses a method for forming a porous film by first forming a block copolymer of a polyimide matrix and a thermally decomposable polymer matrix, forming a film therefrom, and then removing the thermally decomposable polymer from the film by heating. The above-mentioned article discloses a process for making a foam by blowing a polyimide with the aid of a blowing agent.
The polyimide resin per se is much refractory to molding, because it is highly chemical resistant and has a glass transition point of 300xc2x0 C. or higher. The disclosed methods for making this resin porous, therefore, have advantages and disadvantages. Now, the problem involved in the use of porous polyimide resin as, for example, a heat insulator will be discussed.
The method disclosed in the above-mentioned Japanese patent publication can produce a fine porous film having a pore size of 1 xcexcm or less. However, the disclosed method has a drawback that it is not applicable to any low heat-resistant base because it includes a heating step for removing the thermally decomposable polymer. The disclosed technology in the above-mentioned article is also disadvantageous in that since formation of cells in the order of several hundred xcexcm is inevitable, it is necessary to minimize the size of those cells in order to improve the heat insulation of the resultant foam. A variety of methods for making the polyimide resin porous have been attempted in addition to the above-mentioned methods.
Under the circumstance, however, these is no established technology for producing a porous material from the polyimide resin, and a technology which satisfies needed usage is experimented if occasion demands. In other words, there is a great demand for a method which can produce a porous material having a low density and a small mean pore size from the polyimide resin.
In view of the above-discussed problems, a primary object of the present invention is to provide a porous material having a low density and a small mean pore size.
Another object of the present invention is to provide a method for producing such a porous material.
A further object of the present invention is to provide a heat insulator exhibiting high insulating performance and a semiconductor circuit with high dielectric characteristics comprising the heat insulator.
In view of the above-mentioned objects, the porous material in accordance with the present invention is characterized by comprising a dry gel of a polyimide resin which has an apparent density of 800 kg/m3 or less and a mean pore size of 1 xcexcm or less.
The present invention relates to a porous material comprising a dry gel of a polyimide resin, said porous material having an apparent density of 800 kg/m3 or less and a mean pore size of 1 xcexcm or less.
The polyimide resin has preferably a three-dimensional crosslinked structure.
Also, the dry gel is preferably an aerogel.
It is preferably that the porous material having an apparent density of 800 kg/m3 or less and a mean pore size of 1 xcexcm or less.
Further, the present invention relates to a method for producing a porous material, comprising the steps of:
gelling a polyimide resin to have a wet gel of the polyimide resin and
drying said wet gel to remove a solvent from said wet gel thereby having a dry gel of said polyimide resin.
In the method, it is preferably that the gelling step comprises a step of forming a solution or a swollen body of a polyimide precursor and a step of imidizing said polyimide precursor in said solution or swollen body.
Also, the gelling step preferably comprises a step of obtaining a polyimide resin by imidizing a polyimide precursor, a step of forming a solution or a swollen body of said polyimide resin and a step of crosslinking said polyimide resin in said solution or swollen body.
Further, the gelling step preferably comprises a step of forming a solution or a swollen body of a polyimide precursor, a step of forming a wet gel of said polyimide precursor by crosslinking said polyimide precursor in said solution or swollen body and a step of imidizing said polyimide precursor in said wet gel.
Also, the present invention relates to a method for producing a porous material, said method comprising the steps of:
forming a solution or a swollen body of a polyimide precursor;
gelling said polimide precursor by crosslinking said polyimide precursor in said solution or swollen body to form a wet gel of said polyimide precursor;
drying said wet gel to remove a solvent therefrom to form a dry gel of said polyimide precursor; and
imidizing said polyimide precursor in said dry gel to form a dry gel of a polyimide resin.
In the above-mentioned method for producing a porous material, the drying step preferably uses an supercritical drying method and a finally obtained dry gel is an aerogel.
In addition, it is preferable that these method further comprises a step of heating said dry gel of said polyimide resin in an inert atmosphere at 500xc2x0 C. or higher to form a porous material comprising a carbon material.
In these method, it is preferable that a finally obtained porous material has an apparent density of 800 kg/m3 or less and a mean pore size of 1 xcexcm or less.
The present invention relates to a heat insulator formed by filling the above-mentioned porous material.
It is preferable that the vessel is a gas-tight vessel and has been reduced in internal pressure.
Further, the present invention relates to a semiconductor circuit formed by using the above-mentioned porous material, said circuit being formed with an interlayer insulating film which effects electrical insulation between a plurality of electrode lines arranged on a base plate.